1. Field of the Invention
The present invention relates to a method of producing elongated large-size forged article from an alloy steel containing, by weight, 0.08 to 0.25% of carbon, 0.02 to 0.15% of niobium and 9 to 12% of chromium and, more particularly, to a method of producing forged article suited to use as the rotor shaft of a steam turbine.
2. Description of the Prior Art
Known alloy steels used as the material of elongated large-size forged article for use at high temperature, e.g. rotor shaft of a steam turbine, contain about 10 wt% of chromium and a small amount of niobium. Such alloy steels are shown, for example, in the specification of the U.S. Pat. No. 3,139,337.
It proved, however, that elongated large-size forged articles of a length greater than 5 m and a diameter exceeding 500 mm, made from an alloy containing small amount of niobium and about 10 wt% of chromium, often suffers serious deterioration in the mechanical property, particularly in the elongation and reduction of area, due to generation of eutectic niobium carbides in the core portion of the cast ingot during the ingot making. It proved also that sedimental crystals containing non-metallic inclusions are formed undesirably in the lower portion of the cast ingot during the ingot making.
In the production of elongated large-size forged article, the ingot is usually formed so as to have a height which is larger than its diameter, because such a form of the ingot reduces the number of steps in the forging process. More specifically, the ratio H/D of the height H to the diameter D of the ingot, neglecting the hot top portion, is selected usually to range between 1.5 and 2.0, measuring the diameter D at the heightwise mid point of the ingot. In this vertically elongated steel ingot, the solidification in the core portion of the ingot takes place last, so that V-segregation and/or shrinkage cavity are formed in the core portion of the ingot, while sedimental crystals tend to be generated in the lower portion of the ingot. When the ingot is made by means of a metal mold, the V-segregation and shrinkage cavity tend to be formed in large ingots having diameter exceeding 500 mm. The shrinkage cavity is referred to also as loose structure.
Since the influence of the V-segregation and shrinkage cavity are eliminated substantially by the forging, hitherto, it has been an ordinary measure to form and use vertically elongated ingot anticipating the generation of the segregation and/or shrinkage cavity.
On the other hand, the portion of the ingot containing the sedimental crystals is cut and removed because such a portion includes large non-metallic inclusions which adversely affect the mechanical property of the product.
Under this circumstance, a new problem has arisen in which the forged article made from an alloy steel containing small amount of niobium and about 10 wt% of chromium disclosed in the specification of the U.S. Pat. No. 3,139,337 undesirably permits during the ingot making the generation of the eutective niobium carbides in the core portion of the cast ingot in which the solidification occurs later than in other portions to adversely affect the mechanical property of the forged article.